Adjustable frequency selective apparatus



March 16, 1954 o. G. VILLARD, JR

ADJUSTABLE FREQUENCY SELECTIVE APPARATUS 2 Sheets-Sheet l Filed March 7 1951 Ag-gj. ATTORNE BYM www

. .aalli March 16, 1954 o. G. VILLARD, JR 2,672,529

ADJUSTABLE FREQUENCY SELECTIVE APPARATUS Filed March 7, 1951 2 Sheets-Sheet 2 4m woo zooo aooa FREQUENCY /N CYCLES PER SECO/V0 FBEQl/E/VCY /N CYCL ES PEB SEGN() d so.

www 4 :I INVENTO R REL/m v: ,sm/60ml? FREQl/f/vcr) ATTORNEY Patented Mar. 16, 1954 ADJUSTABLE FREQUENCY SELEQTIYE APPARATUS oswaia c. lviinra, Jr.,.ra1o Alto, salir., signor to Theloard tof `Trustees .of .the l Lelaml Stan- ...fcrd Junior University, Sianfscrd Ueli-Yeltsin Application March 7, 1951, SerialNo.-2 14,405

1.2 .Claims- 21 The present invention relates to frequencyselective circuits, and is particularly concerned with a 4circuit na-r-rangement ier-variable hand width and variable selectivity-as well -asyar-iable response frequency. v

v'object of this invention is to `provide improved 'frequency selective circuit-apparatus, for iiexi'bility las-'to band Awidth and selectivity, -as well v4as Aadjustability over a Wide range oi response -frequencies 'it is a further object to provide selective circuit-,apparatus v-of -very high stability.

-Ttfis yet a yfurther object to-'providefrequencyselective apparatus free from reliance upon inductance-capacitance resonant circuits, Vand capable ofadjustability as tof'band width -without cha-nge in the-frequency of-maxirnum response.

Yet another object is to provide selective circuit vapparatus avoiding reliance -upon inductance-capacitance resonant circuits, and capable of tuning over a relatively wide range of 'frequencies, with minimumchange of response LJfrequency'band width.

Selective amplifier systems have heretofore been constructed wherein a vwide-'band `amplier was provided with a feed-.back circuit including a plurality of cascade phase shifter stages. The phase-Shifters were each provided with a phase inverter anda 4series resistance-capacitance v output circuit, -so arranged Yas Ato provide Yan output Voltage of substantiallyconstant amplitude Aand of phase variableoverarange of substantially 18'0 degrees, from.a..condition.of .substantial co.- phasality withthe .individual phase shifter input voltage to a condition of substantial .phaseopposition therewith. l

The `phase Shifters .were Adesigned forequal phase'shifts, andeitherthe capacitorelements or the resistor yelements .were .ganged together for wide range Variation, with equality of the varied elements in the respective stages being maintained. These phase .Shifters could be s o coupled to the output and inputcircuits of the amplifier as to yield positive feedback (regeneration) when their shifts were 90 V each and their Atotal phase shift accordingly 180. With this connection, the

gain of the amplifier .was greatly enhanced at the frequency at whichthe phase shift of veach of the two shifters was 90,-and hence the system was usable as a highly selective tuned amplifier. I f k suflicient feedback and gain were provided, the.

phase shift, thevtwo .outputs cancel, thereby giving zero net transmission.

AThe yfeatures Aof fthe 'above-described system are set for-thin detail :in .a paper by the present in.- ventorentitledifilunable A-F Ampliier, in elec.- tronics, July, 1949, .pages f77-:7.9, and `described and claimed in application Serial No. 153,065, led in -1950vby the present inventor. A closely related system, wherein .the feedback circluit with vtandem '90 phase rshifters is .employed to generate oscillations, vis described .and claimed .in U. S. Patent No. v2,452,586 to Rawley D. utlicCoy.r

According to the present invention, an improved selective `anfiplii'ier :system is made .by .providing dual feedback circuits having their ,output components supplied-in opposition to .each other in the amplifier input circuit; and :in .one of these `feedback circuits, Ithe Aplural phase shifter stages are vconnected 1in tandem `and 4are arranged-for differential control of .their phase shift characteristics. With this arrangement, the .user is enabled to ibroaden the response curve of the system as desired, and withal, to preserve -veryhi-gh lstability of `the system, with particular advantage =in the use-thereof lfor selective iacceptance of a desired` trequency orlfreq uencies-iAn-a-predetermined'band.

In the accompanying drawings,

Fig. :1 -is a circuit diagram ofa preferred 4ernbodiment of the `presen-t invention;

Fig. ,-2 shows a. comparison among `,the -phase shift curves for a phase shiiteroperating `with a 90 Apghaseshi-it, aphase shifter (operating at a very small phase-shift, and afphase shifter operating at a very-large :phaseshifu `showing-the contrast of the-slopes of the vcurves at frequencies in the vicini-ty of vthe operating tr-equency;

Fig. -Bis a T graph -showinggthevarious response versus frequency `curves available through the operation of switch SL97, Mil in Fig. 1; and

Fig. 4 yis a seriesboi response graphs showing the extent-to which the -variation -of rabsolute band twidth witliir-'equenc-y of tuning-is reduced by reliance upon `singie-stage phase shift adjustments inlftheapparatus ofdigwhswitch lI-H be- 21er@ Connected-to theV amplifier, :the .grid

being-hv-passesiftogeround thrgughfthe Capacitor 29, .and arathee bias cireiitiul dhs@ .resistor 3l and Capacitors@areiincluded-1nfthe,empler Staee- .The,circuity constants asscciatedlwiththe ,pen-

",N a tode tube are selected to provide substantially fiat frequency response of this stage over a very wide band of frequencies.

As will become apparent from a consideration of the system of Fig. 1, the use of a single amplier stage l, and the selection of a pentode therefor, are matters merely of design; a plurality of stages may be provided, and triodes or other amplifier elements may be employed if desired.

A dual-channel feedback system is provided between the output terminals l5 and the input circuit I3, l1 of the amplifier One feedback channel employs tube 4| operating as a cathode follower stage with cathode load resistor 43, followed by tube 45, connected as a further cathode follower stage.

The other channel employs tube 4| with its cathode resistor 43 and a plate load resistor 41, being equal, providing phase inverter operation of this tube with balanced output between cathode and anode. Between the cathode and anode of stage 4| is connected a resistance-capacitance circuit for providing an output voltage at terminal 49 whose phase is dependent upon frequency and upon the resistance and capacitance values in the series circuit between the cathode and anode of tube 4|.

This second channel includes a further phase shifter stage 5I which is generally similar in its circuit arrangement and operation to phase shifter stage 4|. Phase shifter 5| is provided with a cathode load resistor 53 and an anode load resistor 51, and with a series resistance-capacitance circuit between cathode and anode, the output terminal 59 of which provides a phase shifter output version of the voltage supplied to the grid of stage 5|.

A dual-triode electronic mixer or combining stage El, 53 is provided, with one triode grid B5 supplied by the output of cathode follower stage 45, and the other triode grid 61 supplied by the second-channel output voltage version developed at terminal 59. The combining circuit 6|, 63 is provided with a cathode load resistance output network including resistors 69, 1|, 13, 15 and 19, the last-mentioned resistor being arranged as a potentiometer-type element for feedback balance adjustment. The output voltage provided at the variable tap of potentiometer element 19 is supplied to a feedback amplitude control circuit including potentiometer 8|, resistor 83 and coupling capacitor 85, which is coupled through resistor 81 to the input circuit of amplier H.

Two selector switches are provided in the resistance-capacitance time constant control circuits of stages 4| and 5|. The rst is a ganged selector switch 9| including two three-position switch sections. At position I, the arms of this switch are connected to equal capacitors 93 and 05 in the respective phase shifter output circuits. When this switch is set to position III, the arms are connected to capacitors of greatly contrasting values, as for example, arm 91 is connected to a capacitor 99 having a capacitance value ten times as high as that of capacitor 93, whereas arm is connected to a capacitor |03 having ik; the capacitance value of capacitor 95. At the intermediate position II, these arms contact terminals connected to capacitors |05, |01, respectively of intermediate values, capacitor being, for example, of three times the capacitance value of capacitor 93 and capacitor |01 being of 1/3 thev capacitance value of capacitor 95.

The second circuit selector feature includes a single-pole, double-throw switch l|| which, in

position A, connects variable resistor section |I3 between the cathode of tube 5| and terminal 59. When the arm of switch is transferred to contact terminal B, this switch removes variable resistor section ||3 from the circuit, and substitutes a fixed resistor ||5 of relatively low resistance value, between terminal 59 and the cathode of tube 5|.

The variable resistor element ||1 connected between the cathode of tube 4l and terminal 49 is a variable resistor similar to resistor element |13 in all respects, and ganged therewith as a dual potentiometer, as indicated by the dotted line connections of the frequency control knob Since the grid of tube 5| is connected directly to terminal 49, with a direct-current path through variable resistor element ||1 to the cathode of tube 4|, which is at a slight positive potential above ground, and since it is advisable that the grid of tube 5| be at negative bias potential with respect to the cathode thereof, the cathode load resistor 53 is preferably of a value appreciably higher than the resistance of cathode resistor 43, e. g. twice the resistance of resistor 43. The resistance values of elements 41 and 43 are equal, and likewise the resistance values of elements 51 and 53 are equal.

Preferably, triodes 45 and 5| are the respective sections of a dual triode tube and likewise triodes 5| and 63 are the respective sections of a dual triode tube.

These dual triode tubes may be of the |2AU1 type, and suitable tube types for amplifier i I and phase inverter stage 4| are the EAUG type tube and the 6C4 type tube, respectively.

Typical resistance and capacitance values for satisfactory operation of the above described system are tabulated below:

Capacitors Microfarad I1 0.1 23 0.1 29 0.1 33 50. 0.1 93 0.002 S5 0.002 99 0.02 |03 0.0002 |05 0.006 |01 0.00066 Resistors 25 0.22 megohm. 21 0.6 megohm. 3l 1500 ohms. 43 1000 ohms. 4B 0-5000 ohms, variable. 41 1000 ohms. 48 2000 ohms. 50 1 megohm. 53 2000 ohms. 51 2000 ohms. 69 5000 ohms. 1| 5000 ohms. 13 3000 ohms. 15 3000 ohms. 19 1000 ohm potentiometer. Si 10,000 ohm potentiometer. 83 0.1 megohm. 81 50,000 ohms. ||5 15,000 ohms. H3, ||1 0-500,000 ohms, each section The first feedback channel, comprising the cathode output circuit of tube il and cathode follower stage 45, supplies its output to the input circuit of tube 6 I. Through the mixer SI, 63, this tends to produce a feedback voltage at the grid I9 of amplifier I I which is in phase with the amplier output voltage supplied to terminals I5. Since amplier I I employs an anode output circuit, its output voltage is 180 out of phase with its input voltage, and accordingly, the above-mentioned feedback voltage supplied by tubes 45 and 6| is an inverse feedback voltage for amplifier II.

Assuming that switch III is set to position A and switch 9| is set to position I, then for a desired frequency of the input Voltage of amplifier II, the tuning control II9 may be adjusted to set the variable resistors II? and IIS' for resistance values equal to the impedance magnitude of capacitors S3 and 95 at that selected frequency. For this setting of control IIS, each of the cascade-connected phase shifters involving tubes 4I and 5I provides 90 output phase shift, and hence, the output voltage provided between terminal 59 and ground is shifted by 180-the sum of the phase shifts of the cascade-connected phase shifter stages with respect to the grid voltage of stage 4I.

This output voltage, which is supplied tothe grid 67 of tube 53, is directly in phase opposition to the cathode output voltage of stage 45, and the voltage component produced across resistor 8| due to the signal at grid 5l is directly in opposition to the voltage component which tubes 45 and 5I tend to produce through the action of tube 6I, this latter voltage being a direct positive feedback or regenerative fedback voltage for the input circuit of amplifier I I.

In the use of this system, variable cathode resistor 46 and feedback balance control resistor 'I9 are adjusted to provide equality of the feedback voltage components produced in resistor 8|, so that under the aforementioned conditions of 180 phase shift between the grid of tube 4I and terminal 59 of phase shifter stage 5I, the respective output components in resistor 8| are cancelled out, and hence no net feedback is provided to the grid circuit of amplifier I I.

For other frequencies, the phase shifts at terminals 49 and 59 are appreciably greater than 90 and 180, respectively, or appreciably lower than 90 and 180, respectively, depending upon the direction of frequency variation from the selected frequency. Under these conditions, the output components in resistor 8| are not cancelled out, but instead, a net feedback is realized in the grid circuit of amplifier II, substantially in phase opposition to the input voltage supplied to terminals I3 thereof, so that the effective over-all gain of the system of Fig. 1 for such frequencies is materially reduced.

Each of the cascade-connected phase inverter stages provides substantially constant amplifier output voltage, so that the realization of zero net output voltage across resistor BI is entirely a matter of phase relationships and not of reduction of amplitude of the current variations through combining circuit tube 63, when the frequency departs from the selected frequency.

Now, the response band Width of the system with the aforementioned connections and adjustments is relatively narrow. It is definitely advantageous to have such sharpness of selectivity available, but it is often desirable to be able to broaden the responsiveness of the amplier system.

For this purpose, at a given operating fre-v quency for which, as described before, the impedance magnitudes of elements I|3 and II'I are equal to the impedance magnitudes of elements and 93, respectively, the operator may reset switch 9|, 91, IOI to position II. With this resetting of this switch, capacitor 93 is replaced in the anode cicuit of stage 4I by capacitor |05 of substantially three times the capacitance value, whereas capacitor $5 in the anode circuit f tube 5I is replaced by a capacitor I0II of substantially one third of the capacitance value. The phase shift of stage 4I, in this case, is made appreciably greater, and the phase shift of stage 5| is made appreciabiy less, the sum of the phase shifts of these stages nevertheless being retained at The amplitude of the voltage at terminal 59 remains constant, because each phase shifter supplies an output voltage of amplitude independent of its phase shift. But the rate of change of total phase shift at terminal 59 with respect to the grid voltage of tube 4I, as a function of frequency of the ampified input signal, is greatly reduced.

This is represented by the curves of Fig. 2, which show the relative steepness of the curve of change of phase with respect to relative changes of frequency in a phase shifter of the resistance-capacitance type set for 90 phase shift, as compared to one phase shifter of the same type set for a very small phase shift, and another phase shifter of the same type set for a very large phase shift. Curve ISI indicates the operation of the 90 phase shift stage, whereas curves |53 and |35 illustrate the operation of stages set for very low phase shift and very high phase shift, respectively.

For even broader band width than provided at setting II, switch 9| may be set to position III, at which position the phase shift of the stage including tube 4I is as represented by curve |35, and the phase shift of the stage including tube 5 I is as represented by curve |33.

Fig. 3 illustrates the respective response vs. frequency curves obtainable at 1G00 cycles for the three band-Width settings of the circuit arrangement of Fig. l. Curve IIiI is the curve of the sharpest frequency response obtainable, illustrating the operation of the system of Fig. l with switch 9|, 97, i0| set to position I. Curve Id illustrates the selectivity characteristic for position 1I, and curve |45 illustrates the broadest response curve, available with switch 9|, 51, IIlI set for position III. `5

By virtue of the opposite changes of the capacitance values about the equal values of capacitors 53 and 95 as the geometric mean, the midband frequency or maximum response frequency for a given setting of snob IIQ remains constant as switch si, t'l', IGI is changed from one band-width position to another. Thus, even while the system is being used to receive a desired signal, the band width may readily be increased or reduced by the operation of switch 9|, without necessitating appreciable readjustment of control I I9.

Normally, with switch 9|, 91, IIiI set to position I for the use of the equal capacitors 93, 95, for maximum sharpness of tuning of the amplier system, the relative band width of the system remains constant. Accordingly, as the control I I9 is rotated in the direction for increasing the resistance values in resistor elements II3 and I I'I, for decreased response frequency of the system, the actual band width in cyclesrbecomes extremelynarrow. Thisnarrowing; band `width efectfmay be offset by the. use of :the system :with .switch vl Ill set tf1-:position -Bf leaving the control'ils eifective tov vary the re- Sistance in only one ofthe two cascade-connectedwphase shifterstages. Now, if the resistor H5 has afrelatively `low resistance value, sothatas knob ii-9 -is operated in the directionofincrease ingresistance, 'it is also increasing the contrast between the timeconstants and the resultant phaseshifts of the two phase Shifteristages, the relative bandwidth is` increased as the `system is Atuned tovlower. frequencies. This operation is illustrated .in theplural curves of Fig. 4, wherein the relative broadeningof the relative response curve for a maximum response frequency of e() cycles is .illustrated :in comparisonto theme diumrband width response curve 'for 1000 cycles andthe relatively .narrow band `width for 5000 cycles.

vThe operation with switchsi set .to position I and switch iii .set to position B isparticularly usefulwhere the `system .of Fig. l is `emplcymi .as a tunable high-selectivity audio ampliersystem in the reception of continuous-wave-tele-- graphic signals. For this purpose, the .apparatus of Fig.. l may be connected to the output ter minals of the audio amplifier of a superheterodyne receiver which includes an internal I. F. beat frequency oscillator. selectivefapparatus, it is desirable that the operator be .able .to adjust control H to bring the selected frequency of the system of Fig. 1 into register with 'the'. frequency of a relatively weak radiotelegraph signal to be received. ff the feature of switch lil and resistor H5 were not available, then as the operator shifted the rsettingzofxcontrol .i I9 for progressivelylower frequencies, theabsolute bandwidth of the system wouldy be so vreduced asto causeexcessive ringing of the radiotelegraph signals, or to render them unintelligible.

As is apparent from Fig. d, however, the .relative band width is so broadened, as the control H9 is tuned to progressively lower frequencies, as toretain ample absolute band width ,of the system.

Since many changes could .be made in the above construction .and .many apparently widely different embodiments of this invention. could be `made without vdeparting from the Scope thereof, it is intended that all matter .containedin the above description or :shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What isclaimed is:

`1. A variable frequency-Selective .amplifier system comprising a broad-band amplifier having anfinput circuit for receiving an input .signal and an output circuit for delivering .an .amplifled output version thereof, first .and second feedback Acircuits connected .between said `output .circuit and said input circuit, said rst .feedback circuit comprising means for supplying at said input circuit a substantially fixed-phasefirst Version of the voltage in said output 4circuit,..and said second feedback circuit comprising .means including at -leasttwo tandem-connected phase Shifters of additive yphase shifts Yfor supplying at said inputcircuit a second version of the voltage in said output :circuit of equal amplitude -to Saidrst version andainl phase opposition thereto at a predeterminedoperating .frequency and .for providing appreciable .phase-yariation of .said second voltage versiones a functionfof-.variation in such .use of the of frequency of theamplinerFinput signal, Said second feedback circuit including means for var-ye.. ingfthefphase shift of one .of said phase .Shifters relative to the other of said phaseshifters.

2. Avariable frequency-selective amplifier system-as defined in claim l, wherein the substantially nxedfphase first voltage version supplied tosaid input circuit by said nrst feedback .circuit .is inphase opposition to the amplifier .inputsignal.

A variable frequency-selective amplifier system comprising abroad-band amplifier having an .inputcircuit forreceiving an input Signal. and an .output circuit for delivering an amplified out.- putversion thereof; first and second feedback circuitsconnected between said output circuit and saidinput circuit, saidirst feedback .circuit comprising means for supplying at said input circuit a.substantially xed-phase first version of .the -voltage in Said output circuit., and said second feedback circuit comprising vmeans in.- cluding. two tandeinf.connected phase Shifters of additive .phase shifts for supplying at said input circuit a -second version of the voltage in said output .circuit of ,substantially equal amplitude to said first versienand in phase opposition .thereto at a predetermined operating frequency and fol. providing. Vappreciable phase variation of. said second .v cltage version as a function .of variation offrequency of the amplier input signal; and means ,for differentially varying the phase shifts in said v.tWO phase shifters.

.4. A variable frequency-.selective amplifier sys,- tem as .denned in claim .3, whereinsaid means for differentially varying the phase shifts in the two phase Shifters comprises means for advancing the output phase of one of the phase Shifters and retarding the output phaseof .the other .and retaining the sum rof the phase shifts .of said phase Shiftersequal to 18.0. at. the ,selected `op.` erating frequency..

5 A .variable band .width frequency-.selective amplier System, comprising. a .broadeband ampli,- er having an inputcirouit for receiving .aninputsignal `and .an output .circuit fordelivering an amplified output version thereof; .and `e feed.- back circuit connected between `said output .circuit and said input circuit including two .tandem-connected phase shifters of .additive ,phase Shifts for supplying vat said input circuit .a .var i.- able-phase version of the voltage ,in said output circuit,4 both of saidphase Shifters being. characterized by 'increasing ,phase shift for changes of freguency'in onesense andby .decreasing .phase shiftfor changes of frequency in the Kopposite sense, means for simultaneously increasingor decreasing'the Iphase shift introduced by .there- Spective Iphase shifters ata given frequency, ,and means Vfor Adifferentially varying .the phase .shift of one of said phase Shifters relative to thenther -ofsadphase Shifters, said .last named means acting. independently of said first named means .whereby the. relative phase shift introduced by each ofthe two. phase :Shifters intandemV is variable .independently .of the total phase Lshiftvof the -phase .Shifters 6. Awarlable vfrecpuency-selective amplifier .system, comprising al broad-band .amplifier .having an `input circuit for -receivingan input Signal and an .output circuit -f or delivering an amplifier .route put .version thereof; .rstand Asecond feedback .circuits .connected between said output circuit .andaid :input .-circuit,.-said first .feedbackl i .CntcQmprisingmeans .for Supplyingat zslgyi'din.

put circuit a substantially fixed-phase flrstiverf sion of the voltage in said output circuit, and said second feedback circuit comprising means including first and second tandem-connected phase shifters of additive phase shifts for supplying at said input circuit a second version of the voltage in said output circuit of substantially equal amplitude to said first version and in phase opposition thereto at a predetermined operating frequency and for providing appreciable phase variation of said second voltage version as a function of variation of frequency of the amplier input signal.

7. A variable frequency-selective amplifier system as defined in claim 6, wherein each of said phase shifters comprises a phase inverter stage having an anode-cathode load circuit including a resistor element and a capacitor element connected in series, the junction between said elements being the output terminal for phase shifted output Voltage.

8. A variable frequency-selective amplifier system as deiined in claim 7, wherein the resistor element in the series load circuit of said iirst phase shifter and the resistor element in the series load circuit of said second phase shifter are ganged variable resistors of substantially equal resistance values.

9. A variable frequency-selective amplifier system as defined in claim 8, wherein said capacitor elements in the series load circuits of said first and second phase shifters are fixed capacitors of substantially equal capacitance values, said variable frequency-selective amplifier system further including means comprising a ganged selector switch having terminals of the respective sections connected to said xed calo f pacitors, and unequal capacitors connected to further terminals of said respective switch sections for differentially changing the capacitances of said phase shifters to reduce the phase-frequency sensitivity of said phase Shifters.

10. A variable frequency-selective amplifier system as defined in claim y"8, further including means for selectively substituting fixed resistance of a relatively low resistance value in the series load circuit of one of said phase shifters, whereby the relative band width of said variable frequency-selective amplifier system is broadened as the resistor element in the rseries load circuit of the other of said phase shifters is adjusted to progressively higher resistance values.

1l'. A variable frequency-selective amplifier system as defined in claim .6, further including means for simultaneously reducing the phase shift in one of said phase shifters and increasing the phase shift in the other of said phase Shifters, by equal extents, at a selected operating frequency.

12. A variable frequency-selective amplifier system as defined in claim 6, further including means for varying the phase-frequency determining element of one of said phase shifters, the phase-frequency elements of the other phase shifter being retained fixed.

OSWALD G. VILLARD, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number 

